It’s hard to say when a writing project is finished, since words can be tweaked for a long, long time and each tweak seems to make an improvement. Even when writing a first-draft, you might think that the author is done when she/he writes ‘the end.’ But the ending often reveals a thread that needs to be strengthened earlier, or a twist that should be explored. So writing a book doesn’t usually end with a flourish, more with a shrug, I find.
Resolving to write 50,000 words in a month does have the benefit of having an end point. So it is possible to be done at a point in time. But thoughts return to writing over the next several days and weeks. And that’s just the first draft. A book isn’t done after the first draft is done, in many ways it is just beginning.
I often think I’m on the last read-through of a novel, just checking for errors, but then I start improving things so that I know I’m on the penultimate version, except that I do that several times.
Basically, it’s hard to know when something is finished.